1. Field of the Invention
The present invention relates to a liquid crystal display device of a passive matrix type and an active matrix type. Particularly, the invention relates to an electrode structure of a transflective type liquid crystal display device having both functions of a transmission type and a reflection type.
2. Description of the Related Art
In recent years, by explosive spread of a portable information terminal represented by a cellular phone, there is needed a display capable of dealing with light-weighted formation, low power consumption and a change in an environment of use.
Further, in views of thin film formation and light-weighted formation, a liquid crystal display device or an organic EL display device is representatively promising.
Power consumption of a transmission type display device is inconsiderable for driving only a display. However, a liquid crystal per se does not emit light and therefore, a back light is needed for displaying as a display. For use of a cellular phone, an EL back light is generally used, however, power is additionally needed for the back light and a specific characteristic of low power consumption of a liquid crystal is not fully utilized, which are disadvantageous in low power consumption. Further, although in a dark environment, display of a display is viewed with excellent contrast, in an ordinary bright environment, the display is not viewed so well and there is a drawback in adaptability in accordance with the environment of use both in cases of an upper emitting type and a lower emitting type.
Further, the organic EL display device is characterized in which a display element per se emits light. Although power consumption thereof becomes larger than that of a reflection type liquid crystal display device, the power consumption is smaller than that of a transmission type liquid crystal display device (having back light). However, similar to the case of the transmission type liquid crystal display device, although in a dark environment, display of a display is viewed excellently, in an ordinary bright environment, the display is not viewed so well and therefore, there is still a drawback in adaptability in accordance with an environment of use both in cases of the upper emitting type and the lower emitting type.
Further, the reflection type liquid crystal display device utilizes outside light from an environment as light for display. On the side of the display, the back light is not basically needed, only power for driving a liquid crystal and a drive circuit is needed and therefore, positive low power consumption is achieved. Further, quite contrary to the former two, although in a bright environment, display of a display is viewed excellently, in a dark environment, the display is not viewed so well. Considering the use of a portable information terminal, the portable information terminal is mainly used outdoors and there is frequently a case of viewing the display in a comparatively bright environment, however, this is still insufficient in terms of adaptability in accordance with an environment of use. Therefore, locally, a reflection type display device integrated with a front light is on sale such that the display can be carried out even in a dark environment.
Hence, attention is given to a transflective type liquid crystal display having advantages of both of a transmission type and a reflection type liquid crystal display device by combining the device. In a bright environment, a characteristic of the reflection type of low power consumption and excellence in visibility under the environment is utilized, meanwhile, in a dark environment, a characteristic of excellence in contrast provided to the transmission type is utilized by using a back light.
A transflective type liquid crystal display device is disclosed in JP-A-11-101992. The device is a reflection and transmission type (transflective type) liquid crystal display device. More concretely, by fabricating a reflection portion for reflecting outside light and a transmission portion for transmitting light from a back light in a single display pixel, in a case where the surrounding is totally dark, as a reflection and transmission type liquid crystal display device, the display is carried out by utilizing light transmitting through the transmission portion from the back light and light reflected by the reflection portion formed by a film having comparatively high reflectance, while in a case where the surrounding is bright, as a reflection type liquid crystal display device, the display is carried out by utilizing light reflected by the reflection portion formed by the film having the comparatively high optical reflectance.
Further, in the above-described transflective type liquid crystal display device, particularly at the reflection portion for carrying out reflection display, a special concavo-convex structure having optical diffusion is given. Since a reflection electrode, according to the structure thereof, reflects light from a certain direction by a certain incident angle only to a location having a specific exit angle in a specific direction (Snell's law) to the surface, when the surface is flat, a direction and an angle of emitting light are determined to be constant relative to incidence of light. If a display is fabricated under such a state, a display having very poor visibility is brought about.
The liquid crystal display device of a transflective type is considered as a display well coped with the particular service conditions for the personal digital assistant. Particularly, in the cellular phone application, huge demand is to be prospectively expected from now on. For this reason, in order to secure stable demand or cope with huge demand, there is an apparent need to make efforts toward the further reduction of cost.
However, in order to form a concavo-convex structure as noted before, there is a need for a method to provide a concavo-convex form in the layer lower than the reflection electrode and then form thereon a reflection electrode.
Meanwhile, in order to fabricate a transflective type liquid crystal display device without limited to the foregoing example, patterning is required for forming a concavo-convex structure in one or both surfaces of a reflection electrode and a transparent electrode configuring a pixel electrode or in the layer beneath the pixel electrode, thus increasing the processes. The increase of processes would incur a disadvantageous situation, including yield reduction, prolonged process time and increasing cost.
Accordingly, it is an object of the present invention to provide a display having high visibility and a transflective type liquid crystal display device having a reflection electrode with a concavo-convex structure formed without particularly increasing the processes.